In a digital printing system a document may be made up of multiple forms, each form may be made up of multiple images, and each image may be made up of multiple image planes. Although these terms are commonly used in the printing industry, the meaning of these terms is not precise and tends to vary. As used herein, a form refers to the contents of a physical portion of a printed media such as a page. A form may have one or two sides. A form origin is a position at the top of the form, while a cue mark is a physical mark or a logical position for pin-feed cueing, identifying the form origin. A document can be defined as a series of forms that are logically associated, such as a book or a multi-page billing statement. A print job can be defined as a series of documents. In some cases, a document may consist of multiple forms where the forms are printed 2-up across a web of paper. The information printed on one side of a form is an image, while an image plane is a “layer” of an image printed by a single print engine. For example, in a processed color job, each of the CYMK colors is printed by a single print engine in a separate image plane. A print engine is a marking device such as a printhead that prints one image plane, or a portion of one image plane. In an Advanced Function Printing/Intelligent Printing Data Stream (AFP/IPDS) environment, each of the Object Content Architecture (OCA) colors is in a separate image plane. An Image Plane Coordination (IPC) Mark refers to a mark associated with the image plane. A Composite Image Group (CIG) Mark is a set of IPC marks consisting of one IPC mark for each image plane in a form.
In some systems, the form width may be larger than the print engine print area and multiple print engines may be stitched together side by side to construct a complete form. A very simple document can be described as consisting of multiple sequential forms, each with a single image plane, printed on only one side of the form. Since such a document is generally printed using a single print engine, synchronization is not an issue in that case. However, many documents are more complicated and require multiple print engines to create each form. Common examples of this are two sided (duplexed forms), forms printed in multiple colors, or forms made up of multiple stitched print engines. It is possible for the print engines to get out of synchronization with each other, or for the data sent to any of the print engines to get out of synchronization, such that images or portions of images are no longer properly synchronized to the proper form. The consequence of incorrect synchronization is generally very significant and measures used to detect the condition are necessary.
Various methods exist for detecting and addressing synchronization. For example, some printing systems print bar codes or other marks containing data on each side of a form, and after the form has been printed, read the codes or marks to insure they are properly synchronized. Existing methods for detecting synchronization require the printing of specialized marks, a reader or a scanner to read the marks, and software to interpret the marks and validate the results. While this system may be suitable for monochrome applications, it becomes very cumbersome for applications where a large number of print engines may be used. i.e. 2-up duplex, four color print job.
It is seen then that there exists a need for an improved means for detecting synchronization, particularly for printing of multi-color applications.